The Edinburgh New Philosophical Journal

and magnesia, are the substances which are found in the residua of the combustion of fossil wood and brown coal. They present themselves in very different and very variable proportions, which depend upon the local circumstances under the influence of which the deposition of matter has been effected in the natural beds of these combustibles.

In black coal, the quantity of charcoal which may be obtained, by means of distillation in the dry way, varies still more than in the different sorts of brown coal, comprising also fossil wood. M. Karsten has not met with any black coal, which, on being distilled, has furnished less than 48 per cent. of charcoal. From this number, the quantity of residuum in charcoal rises to 90 per cent. Between these two limits there is scarcely a number to be found that would not answer for the produce in charcoal, or coke, of some kind of coal. Striking differences, however, are remarked in the external form of the carbonized coals called cokes.

In some the form of the coal remains unchanged, the volume only being diminished, as in charcoal from fresh vegetable fibre, fossil wood, and brown coal. Others remain unchanged in form and volume, while some swell and expand more or less. In order to observe correctly these different relations, it is necessary to use the coal we intend submitting to dry distillation in the state of powder. Coal of the first kind affords a coke in a dusty pulverulent state, without the least cohesion, just as in brown coal. In coal of the second kind, the powder is conglutinated into a cake, often very solid and tough, but without any swelling or intumescence. The fine powder, in coal of the third kind, melts, and forms a homogeneous mass, which takes the form of the retort in which it is distilled, and frequently swells so much as to choke up the retort.

Here the author divides coals into three classes, which he establishes from the external appearance of the charcoals or cokes which are produced by them. For the object which he -proposes to himself, M. Karsten distinguishes,

1st, The coals with pulverulent coke, (Sand Kohlen) ; 2dly, Those with conglutinated coke, (Sinter Kohlen); and, 3dly, Those with an intumesced coke, (Back Kohlen).

These three denominations sufficiently indicate the aspect and mode of existence of each of the three sorts of coke, as well as the transition which may take place from one kind to the other.

In all these kinds of coal, as in unaltered vegetable fibres, the quantity of charcoal obtained, differs according as a slow or quick heat is employed during distillation. In general, this difference of product is so much the greater, that the coals contain less charcoal. The coals with intumesced coke, however, form an exception. These often, with a greater quantity of charcoal, present greater differences of product in the two modes of carbonization, than with a less quantity of charcoal the coals with pulverulent coke do, and especially than those with conglutinated coke. At the most, these differences of product, in all the varieties of coal examined by M. Karsten, do not exceed 6 per cent., and even this maximum of difference was only observed in a coal with an intumesced coke, which presented a mean quantity of charcoal. The produce in coke of coals of this class, when they possess a greater quantity of charcoal, does not vary more than 4 per cent. in the two modes of carbonization.

Another remarkable fact is, that the application of a low heat, raised very slowly to the strongest red heat, diminished in coals the property of furnishing either a conglutinated or an intumesced coke. A coal which, on being subjected to a rapid incandescence, announces itself as belonging to the second class (coal with conglutinated coke), may, by means of a heat raised very slowly, present the aspect of a coal of the first class (that with pulverulent coke). It is chiefly in the transitions from the one to the other class, that this fact is observed. In like manner, by means of a slow heat, a coal of the third class presents the aspect of the second, and especially if the coal in question possesses only in a feeble degree the property of furnishing an intumesced coke. In every case, if the heat be produced but slowly, the swelling of the coals with vesicular coke is diminished. They then form a less loose, less bulky, and less light mass, than if an ardent heat had been rapidly applied.

A distinction between the coals which swell, and those which do not, has long been established in the arts, because these twokinds of combustibles act very differently. Manufacturers have readily observed the great influence which the manner that dif

ferent kinds of coal have in comporting themselves, exercises over their use. They have remarked, that the coals which swell cannot always be substituted by those which do not, and the reverse. But, between the one and the other, common opinion establishes no other difference than the following:-The coals, it is said, which swell, are only distinguished by a greater quantity of constituent parts, which are not carbonaceous, parts which have been designated by the name of Bitumen; in other words, it is the quantity of charcoal which decides whether a coal possesses the property of swelling or not.

case,

This opinion is incorrect; and, so far from this being the it is most commonly observed, that the quantity of charcoal is greater in those coals which swell, than in others. There are coals of the first and second classes (with pulverulent and conglutinated coke), which, on being carbonised, do not yield more than about 50 per cent. of coke, and very few coals of the third class (with intumesced coke) yield so little. On the contrary, a great number of these coals with intumesced coke, furnish upwards of 80 per cent. of a very loose and swollen coke. Such a coal cannot contain so many constituent parts, which are not charcoal, as a coal with pulverulent or conglutinated coke, from which there is only obtained about 50 per cent. of coke.

The products of the distillation of coal in the dry way are well known. The greater the quantity of charcoal, the thicker is the consistence of the oil which is formed. All the varieties of coal, without exception, on being subjected to dry distillation, give feeble traces of ammonia. The coals with pulverulent coke, when they have a small proportion of charcoal, present traces of an acid. In all the varieties of coal belonging to this first class, the proportion which the aqueous fluid bears to the oily fluid, is greater than in those of the second class; and, in these latter, the proportion is greater than in the coals of the third class (those with intumesced coke). The quantity of gaseous substances, and of fluids or vapours which is formed, is in the inverse ratio of the contents in charcoal. A smaller quantity of gas is disengaged by the varieties of black coal, than by most of the brown coals; but, in the former, the combinations of carburetted hydrogen are more predominant. Sulphuretted hydrogen gas is only formed when the coal is mixed with iron pyrites, which it

very generally is. The more capable the coal is of swelling (the third class), the more does the proportion of oil gas increase in the gaseous mixture.

It is only in those coals of the first and second classes in which the quantity of charcoal is small, that a decomposition of the combustible is effected before it has experienced a red heat; and even in these coals the decomposition does not make a marked progress at a low temperature.

The oily substance never begins to be developed until the heat has attained the degree of deep red. To all the coals of the two first classes, as well as those of the third, which contain much carbon, a low red heat must be applied to begin the decomposition, and a very strong red heat to terminate it. All the varieties of coal, besides oil and gas, also disengage water, on being distilled in the dry way.

In the ordinary trials of coals, the object of which is to determine the quantity and kind of coke or charcoal which they are capable of furnishing by dry distillation, the coals are usually employed in a state of desiccation in the air. This method is sufficient for common purposes; but it does not answer for chemical analysis, properly so called. In this latter case, M. Karsten found it necessary to dry, at the temperature of boiling water, the various combustibles which he intended to analyse chemically, with the view of comparing the results of the analysis, with the effects which the same coals produce on being submitted to dry distillation.

The author had at first presumed, that all varieties of coal, taken in their ordinary state of desiccation in the air, and such as they are employed for carbonisation, would not undergo a great loss of weight at any temperature below that of boiling water; or that, at least, this loss of weight would be nearly equal in all. But in order to attain his object, he found himself obliged to enquire what loss of weight coals experience from desiccation, at the temperature of boiling water. Hence a series of comparative trials which M. Karsten also extended to some other substances.

All these matters reduced to powder, were first exposed during five days, under the same circumstances, to a temperature of from 11 to 12 degrees of Reaumur's thermometer. When they JANUARY-MARCH 1827.

were all thus reduced to the same degree of desiccation, an equal quantity of each of them was weighed, and then dried at the temperature of boiling water; the matter, while still warm, was afterwards weighed a second time, and the difference of weight ascertained. At this high temperature, no decomposi tion of the bodies under trial had taken place, as was proved by all these substances resuming their original weight, after being exposed to the air for 36 hours.

The following table shews the weight, after desiccation, at the temperature of boiling water, of several substances experiment